Materials science

Safe deposits

A new way to build complicated materials, micron by micron

MATERIALS scientists have long wished that they could design and build their products in the same way that architects and engineers design and build new houses and cars. When erecting a house, its architect does not make an enormous house-shaped mould, fill it with concrete and leave it to dry. Rather, houses are built up in pieces: walls with spaces for windows are put into place, then the windows are added, and finally the roof.

If it were possible to craft composite materials in the same way—precisely specifying the distribution of different components within the composite—much more sophisticated substances might be created. And if a prototype material were found faulty in some way (liable to crack, melt or bend, for example), then its structure could be tweaked, rather as an architect might replace or move a weak supporting beam in a house with a wall in danger of collapsing.

Such a thing may now be possible. A technique developed by Jennifer Lewis and James Smay of the University of Illinois at Urbana-Champaign, and Joseph Cesarano of Sandia National Laboratories in Albuquerque, New Mexico, should allow materials scientists to exercise just this sort of control in the future, using colloids.

Colloidal suspensions contain particles of one material (a solid or a liquid) dispersed in another (which can be solid, liquid or gaseous). The radii of the suspended particles are measured in millionths of a metre, so if they can be controlled, fine structures can be built up. As the team reports in a forthcoming issue of Langmuir, a journal devoted to the study of colloids, the new technique allows them to build composite materials with custom-designed features as tiny as the diameter of a human hair.

To achieve this, the team modified an older technique known as robocasting, which was developed by Dr Cesarano. A robocaster is like an ink-jet printer, except that the liquid is extruded continuously as a filament, rather than being sprayed as droplets. The filament is designed to harden as it dries, so an object can be created layer by layer. The catch with robocasting, a manufacturing method that has been used mostly for making new sorts of ceramic, is that the filaments tend to lose their structure and blend together as they dry. This is fine for making uniform materials, but not so good if detailed structure is required—which is where the colloids come in.

Dr Lewis and her colleagues used their knowledge of colloids to create “inks” that retain their shapes as they harden, rather than merging with their neighbours. That makes it possible to create minutely filigreed structures, such as concentric rings and scaffolds. Since the robocasting machine allows the researchers to manipulate several inks at once, they are able to make new kinds of composite materials this way—interlacing the filigrees, and even leaving holes in the material if that helps to arrive at the desired properties.

At the moment, the inks are made of suspended ceramics, including silica and also hydroxyapatite—the primary constituent of bone. The next step for Dr Lewis and her colleagues is to expand their palette. Anything that can be made colloidal (for example polymers, semiconductors or even metals) could, Dr Lewis reckons, be deposited with sufficient precision to be incorporated into a composite. If they prove a success, these experiments could spur the invention of a host of new designer materials.